In CT systems, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient, and impinges upon a linear array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object. Each detector of the linear array produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
The x-ray source and the linear detector array in a CT system are rotated with a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
A reduction in scan time can be achieved by translating a patient in the z-axis synchronously with the rotation of the gantry. The combination of constant patient translation along the z-axis during the rotation of the gantry and acquisition of projection data is known as helical scanning. Helical scanning is described, for example, in U.S. Pat. No. 5.233,518 which is assigned to the present assignee. In addition to reduced scanning time, helical scanning provides other advantages such as better control of contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
Helical image reconstruction algorithms include constant weighting and variable weighting approaches. Constant weighting refers to the weighting schemes in which the weights do not change as a function of the detector channels. Therefore, for each view, a scalar is multiplied to each element in the projection. A variable weighting scheme applies weights that are a function of both the view angle and the detector channel. The majority of helical reconstruction algorithms utilize variable weighting schemes.
In general, the constant weighting schemes are simpler to implement and the data is easier to process. The image quality generated by constant weighting algorithms, however, may be somewhat inferior to that produced by the variable weighting schemes. On the other hand, the variable weighting schemes are more complex to implement. Since the weights are channel dependent, the weighting step and the filtering-backprojection steps are not interchangeable. That is, a backprojection of the filtration of the weighted projection does not equal to the scaled version of the unweighted filtration and backprojection. Therefore, in order to generate an overlapped image in a variable weighting scheme, the projections have to be re-filtered and backprojected.
There exists a need for a helical image reconstruction algorithm that produces images having a quality equivalent or close to the images produced by variable weighting schemes but is simple to implement as with the constant weighting schemes.